The firing rate of a machine gun is determined in major part by a gas block. The gas block redirects a portion of combustion gases from the barrel to the operating group for quickly chambering the next round. The machine gun's ability to sustain a high rate of fire makes it an effective combat weapon.
When a machine gun is used with a sound suppressor, the back pressure from the suppressor elevates the normal cyclic rate of fire but reduces the reliability of the firearm because of that elevated rate. There needs to be a way to preserve reliability of the machine gun when a suppressor is used.